n, k = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))

if n == 0:
    print(0)
else:
    t = [0] * n
    if n >= 2:
        t[1] = t[0] + a[0] + b[1]
    for j in range(2, n):
        option1 = t[j-1] + a[j-1] + b[j]
        option2 = t[j-2] + a[j-2] + b[j] + k
        t[j] = min(option1, option2)
    print(max(t))